Eddy current inspection is one of several non-destructive testing (NDT) methods that use the principle of “electromagnetism” as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle. One of the features of NDT is that it enables a material to be examined without causing damage thereof.
Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path.
Eddy currents are affected by the electrical conductivity and magnetic permeability of materials. Therefore, eddy current measurements can be used to sort materials and to tell if a material has seen high temperatures or been heat treated, which changes the conductivity of some materials.
Eddy current testing has been used as the primary non-destructive evaluation technique in a number of industries for more than fifty years. Though the technique is powerful in principle, the inspection process is often time consuming since the impedance plane signal returned by the probe is difficult to interpret and often requires special operator training.
Research is currently being conducted on the use of a technique called pulsed eddy current (PEC) testing. This technique can be used for the detection and quantification of corrosion and cracking in multi-layer aluminium aircraft structures. Pulsed eddy-current signals consist of a spectrum of frequencies meaning that, because of the skin effect, each received pulse signal contains information from a range of depths within a given test specimen. In addition, the pulse signals are very low-frequency rich which provides excellent depth penetration.
Most eddy current systems use single or multiple coils. The coils are normally excited either singly or in multiples where distance allows. The coils are normally excited with sinusoidal signals and are arranged in a bridge configuration. The detector measures amplitude and phase. The coils can be single or a differential pair. There are many examples of prior art where the sensor design is created as a mechanical arrangement.
US 2008/0068008 suggests the use of spread spectrum in the context of creating a magnetic sensor for crash detection in automobiles. The sensor generates an eddy current at either a single sinusoidal frequency or multiple frequencies by for example spread spectrum or frequency hopping techniques, the purpose being to detect a change in the generated eddy current as a result of an impact.